The present invention relates to the production of aqueous dispersions that contain pyrogenically produced silicon dioxide, a process for their production, and the use of the dispersions for polishing semiconductor substrates.
Cerium oxide dispersions, obtained by calcining a cerium compound followed by grinding of the oxide, have been used for a long time for polishing glass. The dispersions have particle sizes of more than 1000 nm and a high content of impurities. They are suitable for the coarse polishing of surfaces, but not for polishing sensitive optical surfaces, semiconductor substrates or integrated circuits (chemical-mechanical polishing, CMP).
Smaller cerium oxide particles are obtained by the so-called hydrothermal synthesis. In this case a cerium(III) salt is converted oxidatively under the influence of temperature and pressure into cerium oxide, which crystallizes out in the form of fine particles. The particle sizes of the cerium oxide obtained according to this process are 80-100 nm (EP-A-947 469), 7-50 nm U.S. Pat. No. 5,389,352) or smaller than 30 nm and larger than 60 nm (U.S. Pat. No. 5,772,780). The disadvantage of this method however is that very dilute cerium(III) solutions have to be used as starting materials.
Mixtures of cerium oxide and silicon dioxide produced by the processes mentioned above are described in U.S. Pat. No. 5,891,205. In this case the cerium oxide has to be deagglomerated in a first step by means of a grinding mill. The cerium oxide dispersion is then mixed in a second step with a silicon dioxide dispersion based on silica sol and can be used to planarite silicon discs.
The disadvantage in this case is the complicated procedure and the costs involved in the production of this finely particulate dispersion. The deagglomeration of the cerium oxide using a grinding mill conceals the potential danger of the incorporation of impurities in the dispersion. Furthermore, it is known that silicon dioxide dispersions based on silica sol have a higher content of impurities than those based on pyrogenically produced silicon dioxide.
U.S. Pat. No. 5,382,272 describes the activation of silicon dioxide particles by adsorption of a few weight percent, preferably 2 wt. %, of a cerium oxide or zirconium oxide dispersion. The particular feature of this process is that the activating effect is said to be obtained simply by grinding a silicon dioxide dispersion with a commercially available cerium oxide or zirconium oxide dispersion. The mixed dispersion that is thus obtained is said to behave like a pure cerium oxide dispersion when polishing semiconductor substrates with a high removal rate, but without leaving any scratches on the surface to be polished. Compared to the remaining processes in the prior art, the costs are minimal and the execution is simple. The particular disadvantages of this method are, as is also described in detail in WO 00/17 282 A1, the production of a dispersion having a narrow particle size range and the difficulty in achieving reproducible polishing results.
As the prior art shows, the extraordinarily high activity of cerium oxide compared to silicon dioxide and other dielectric oxides has excited a great deal of interest, mainly in the area of chemical-mechanical polishing (CMP).
It is therefore an object of the invention to easily produce an aqueous, cerium oxide-containing dispersion that contains only a small proportion of impurities, and in which the particle size can be reproducibly adjusted.
The above and other objects of the invention can be technical object can be achieved by using an aqueous dispersion containing pyrogenically produced silicon dioxide doped with cerium oxide, in which the cerium oxide is introduced through an aerosol of a cerium salt solution or suspension, and where the mean particle size in the dispersion is less than 100 nm.
In the production of the silicon dioxide doped with cerium oxide a cerium salt solution or suspension is used as starting material, which is homogeneously mixed as an aerosol with the gaseous mixture obtained from the flame oxidation or flame hydrolysis of a volatile silicon compound such as for example silicon tetrachloride, hydrogen and oxygen. The aerosol/gaseous mixture is allowed to react in a flame. The resulting pyrogenically produced silicon dioxide doped by means of the aerosol is separated in a known manner from the gas stream.
As is described in detail in DE 196 50 500 A1, this production process leads to silicon dioxide particles doped with cerium oxide. The cerium oxide particles according to this process are always smaller than the silicon dioxide particles. This is an important prerequisite for the use of dispersions containing the cerium oxide-doped silicon dioxide particles for polishing applications, since otherwise scratches could be formed on the surface to be polished.
The dispersion has a good stability and is ideally suitable for polishing in the CMP sector. Good stability of the doped dispersion means that the time after which the viscosity of the dispersion increases or the dispersion gels or sediments is later than in the case of a dispersion containing a mixture of undoped silicon dioxide and cerium oxide.
The present invention will now be described in further detail.
As a result of the pyrogenic production process, in which as is known highly pure starting materials are used, the levels of impurities relevant for CMP applications are negligibly small.
With commercially obtained cerium oxide dispersions the cerium oxide is generally obtained from ores having a relatively high content of impurities.
The degree of doping of the silicon dioxide produced by the above process may be varied within wide limits from 0.00001 up to 20 wt. %. When used for the production of an aqueous dispersion according to the present invention, the degree of doping is preferably between 10 ppm and 10 wt. %, particularly preferably in the range between 300 ppm and 5 wt. %.
The BET surface of the cerium oxide-doped silicon dioxide is between 5 and 600 m2/g according to a preferred embodiment of the invention. A range between 50 and 400 m2/g is particularly preferred. Within this range the dispersion exhibits a good stability.
The solids content of the dispersion containing cerium oxide-doped silicon dioxide is primarily governed by its intended use. In order to reduce transportation costs a dispersion with the maximum possible solids content is aimed for, whereas for certain applications, such as for example the polishing of silicon discs, dispersions with low solids contents are used. A solids content of 0.1 to 70 wt. % is preferred according to the invention, the range between 1 and 30 wt. % being particularly preferred. In this range the cerium oxide-doped dispersion exhibits a good stability.
The dispersion may be used as such, for example for polishing. The pH of this dispersion is between 3.5 and 4 depending on the degree of doping. The pH may however be varied in a wide pH range of ca. 3-11 by adding basically or acidically acting substances.
An increase in the pH value may preferably be achieved by adding alkali hydroxides or amines. Ammonium hydroxide and potassium hydroxide are particularly preferred.
By adding acids the pH can be displaced to the acidic range down to pH 3. There are preferably used for this purpose carboxylic acids of the general formula CnH2nxe2x88x921CO2H where n=0-6 or n=8,10,12,14,16, or dicarboxylic acids of the general formula HO2C(CH2)nCO2H where n=0-4, or hydroxy-carboxylic acids of the general formula R1R2C(OH)CO2H where R1=H, R2=CH3, CH2CO2H, CH(OH)CO2H or glycolic acid, pyruvic acid, salicylic acid or mixtures of the aforementioned acids. Acetic acid, citric acid and salicylic acid are particularly preferred for this purpose.
The present invention also provides a process for the production of the dispersion containing cerium oxide-doped silicon dioxide, which is characterized in that the cerium oxide-doped, pyrogenically produced silicon dioxide is dispersed in aqueous solution by means of a high energy input. Dispersion methods in which a sufficiently high energy input permits a dispersion of even extremely hard and highly aggregated materials are suitable for this purpose. Such methods include systems operating according to the rotor-stator principle, for example Ultra-Turrax machines, or agitator ball mills. Both systems however have a relatively low energy input. Significantly higher energy inputs are possible with a planetary kneader/mixer. The effectiveness of this system is however dependent on a sufficiently high viscosity of the processed mixture, in order to impart the high shear energies needed to separate the particles.
When grinding and dispersing doped oxide particles there is the danger that the doping substance will separate out during the grinding and dispersion. This leads to variable polishing results if the dispersion is to be used as a polishing agent in the CMP process.
It has now been found that aqueous dispersions containing cerium oxide-doped silicon dioxide particles that are smaller than 100 nm and in which the doping substances has not separated out can be obtained by using high-pressure homogenizers.
With these devices two pre-dispersed suspension streams under a pressure of up to 3500 kg/cm2 are expanded through a diamond nozzle. Both dispersion jets strike one another exactly and the particles undergo a mutual grinding. In another embodiment the pre-dispersion is also subjected to a high pressure, but in this case the particles collide against reinforced wall regions.
These devices have been used for a long time, among other things to disperse undoped oxides such as zinc oxide, silicon dioxide or aluminium oxide (UK-A-2 063 695, EP-A-876 841, EP-A-773 270, WO 00/172 282 A1). The grinding and dispersion of doped oxides using these devices has not been described up to now.
The invention also provides for the use of the aqueous, cerium oxide-doped dispersion for the chemical-mechanical polishing (CMP) of semiconductor substrates or layers applied to the latter. By using the cerium oxide-doped dispersion, a surface that is free of microscratches can be obtained at a high polishing rate.
The cerium oxide-doped dispersion according to the invention is particularly suitable for the final chemical-mechanical polishing in the shallow-trench isolation process (STI process), in which after applying a silicon nitride layer insulating structures are etched in the silicon layer of a wafer, these cavities are then filled with silicon dioxide, and excess silicon dioxide is removed by chemical-mechanical polishing.
These dispersions are also suitable for the production of very finely particulate surface coatings in the paper-making industry or to produce special glasses.